1. Field of the Invention
The present invention relates to a frame of an electroacoustic transducer such as a speaker, and also relates to a method of making such frame.
2. Description of the Related Art
Referring to FIG. 1 of the accompanying drawings, a conventional electrokinetic speaker is illustrated in a cross sectional view. This speaker is an example of electroacoustic transducers. The electrokinetic speaker includes a pole yoke 1 which projects from a center of a back plate. A magnet 2 is placed around the pole yoke 1. A top plate 3 is located on the magnet 2 such that a magnetic gap is formed between the pole yoke 1 and the top plate 3. Accordingly, a magnetic circuit is defined in the speaker. The top plate 3 is firmly secured to the frame 5. A voice coil bobbin is oscillatably located in the magnetic gap. A voice coil 4 is wound around the voice coil bobbin. The voice coil bobbin is supported by a damper 7. A truncated cone-shaped diaphragm 8 is attached to the voice coil bobbin at its center. The diaphragm 8 has a center cap 6 to close the truncated portion. An outer periphery of the diaphragm 8 has an edge 9, which is supported by the frame 5. A lead of the voice coil is connected to a terminal attached to a lateral face of the frame 5 via a cord.
The speaker frame 5 supports the diaphragm 8 and the magnetic circuit such that relative positional relationship between the diaphragm 8 and magnetic circuit is maintained. A periphery of a front portion of the frame 5 is fixed to a baffle plate or a cabinet. In this manner, the frame 5 serves as a fundamental structural member of the speaker system. The electroacoustic transducer frame 5 has to have rigidity and creep resistance. In particular, the speaker frame 5 must be lightweight if installed in a vehicle.
A conventional material for the speaker frame is, for example, steel plate or aluminum (die-cast). The steel plate, however, does not have a sufficient freedom in shape (shaping) so that only limited shapes are available for the speaker frame. Further, a specific weight of the steel is large. When the speaker frame is fabricated by the aluminum die-casting, more freedom is obtained in the shaping but a product (speaker frame) becomes very expensive. A specific weight is also large. In recent times, therefore, a thermoplastic synthetic resin is often used in an injection molding process to make a speaker frame. In particular, the injection molding process is frequently utilized with the thermoplastic resin when speaker frames to be installed on vehicles are manufactured because the on-vehicle speaker frames must be lightweight and fabricated in a large quantity. The injection molding is also advantageous since the speaker frame can be fabricated (molded) with other parts at the same time. This reduces a manufacturing cost.
The resin-made frame for the conventional speaker is satisfactory with respect to the “lightweight” and “reduced cost”, but the thermoplastic resin cannot provide sufficient mechanical properties such as rigidity and creep resistance. In general, therefore, an inorganic filler (e.g., glass fibers, carbon fibers, talc, mica or whisker) is added to the thermoplastic resin (e.g., ABS, polycarbonate or polypropylene). The thermoplastic resin is a base material.
If the resin frame should be lightweight, the base material resin should have a small specific weight. One example of such resin is an olefin resin. If importance is put on an acoustic property, polypropylene is appropriate because its internal loss is great.
Polypropylene is, however, a crystal resin so that it requires a high concentration of filler (e.g., 40% or more) in order to reduce secondary shrinkage (contraction) and increase rigidity. The secondary shrinkage is shrinkage which occurs after an environmental test.
When the filler is added to the resin in a large concentration and an electroacoustic transducer frame is made from the resin-filler material, a molded product (i.e., frame) becomes heavy since the specific weight increases. In addition, the internal loss decreases so that the frame cannot sufficiently absorb (damp) unnecessary vibrations of the neighboring parts and the speaker itself. Furthermore, fluidity of the molten thermoplastic resin deteriorates so that mass productivity of the resin frames and freedom of the shape of the resin frame are limited.